|  | 1.4 | Network Layer Functions
| 1.4.4 |
Why l ayer 3 addresses must contain both path and host information
For path communication to be truly practical, a network must consistently represent the paths available between routers. Each line between the routers in Figure has a number that represents the subnetwork address that can be used by a routing process. The network address contains both a path and a host portion. The path portion identifies a path part used by the router within the network cloud; the host portion identifies a specific device on the network. The router uses the network address to identify the source or destination network of a packet. Figure shows three network numbers coming from the router and three hosts sharing the network number 1. For some network layer protocols, a network administrator establishes this relationship by assigning network addresses ahead of time according to a network-addressing plan. For other network layer protocols, assigning addresses is partially or completely dynamic. The consistency of Layer 3 addresses across the entire network also improves the use of bandwidth by preventing unnecessary broadcasts. Broadcasts cause unnecessary traffic and waste capacity on any devices or links that do not need to receive the broadcasts. By using consistent end-to-end addressing to represent the path of media connections, the network layer can find a path to the destination without unnecessary use of devices or links on the network.